The Met police's Mark Simmons will be questioned by the home affairs select committee behind closed doors. Photograph: Felix Clay for the Guardian

The Metropolitan police is to be allowed explain to MPs in private why it threatened to invoke the Officials Secrets Act in an attempt to force the Guardian to hand over notes and reveal sources behind its phone-hacking coverage.

The hearing on Friday is both official and secret, leading to condemnation by campaigners for media freedom, which threatens to dash the Met's hopes that the furore over their alleged attempt to strike at media freedom will die away.

On Tuesday, the Met announced it would end its attempt to get a court order forcing Guardian reporter Amelia Hill to hand over notes and reveal sources behind the revelation that Milly Dowler's phone had been hacked by the News of the World. It then said MPs had invited them to give a briefing in private. it was not clear who had asked for Friday's session to be held behind closed doors.

John Kampfner, chief executive of Index on Censorship, said: "Holding this hearing behind closed doors would be a damaging move for parliament and the Metropolitan police. It is important that the police explain their actions openly.

"The attempt to use the Official Secrets Act on a journalist was an outrageous attack on free speech and those responsible should explain themselves not just to parliament but to the country."

The powerful home affairs committee has already investigated hacking and lambasted the Met for its failings.

Keith Vaz, chair of the committee, said: "I have asked the Metropolitan police to give the committee a full explanation of why they took this action and to provide us with a timeline as to exactly who was consulted. It is essential that we get the full facts."

The attempt by Scotland Yard to get a production order, requiring the Guardian to hand over sensitive material, was roundly condemned by other news organisations – the first crisis for incoming commissioner Bernard Hogan-Howe, who formally takes office next week.

The Met insists the decision was taken by a relatively junior officer in its directorate of professional standards, who was investigating a detective accused of leaking information from the hacking investigation to the media. The Met says Hogan-Howe knew nothing of the original decision, even though he is overseeing the investigation into phone hacking in his role as deputy commissioner.

Simmons used a radio interview to admit that invoking the Official Secrets Act in an attempt to make the Guardian reveal its confidential sources for stories relating to the phone-hacking scandal was "not appropriate".

Simmons told the BBC: "We've acknowledged and I've acknowledged the role the Guardian has played in the history of what brought us to where we are now, both in terms of its focus on phone hacking itself and indeed its focus on the Met's response to that.

"But in all the glare that has been thrown on to our relationships with the media, we have had to ask ourselves the question about how do we do more to ensure that public confidence in our officers treating information given to them in confidence appropriately is maintained. That's why we undertake robust investigation into incidents of leakages.

"I think what's happened is it's thrown into the spotlight the difficulty that we have in getting a new balance in our relationship with the media in the wake of all the issues that have been aired, very publicly, in recent months."

Alan Rusbridger, editor-in-chief of the Guardian, said: "I just hope that, in our effort to clean up some of the worst practices, we don't completely overreact and try to clamp down on perfectly normal and applaudable reporting. This was a regrettable incident, but let's hope it's over."

The police application was formally made under the Police and Criminal Evidence Act, but with a claim that Hill had committed an offence under the Official Secrets Act by inciting an officer from Operation Weeting – the Met's investigation into phone hacking – to reveal information.